Because manga is one of the few places to read *spoken* Japanese? (For those of us who have limited opportunities to speak to actual Japanese people, it's nice to see the spoken language written down.)

"Give me a fruitful error any time, full of seeds, bursting with its own corrections. You can keep your sterile truth for yourself." -- Vilfredo Pareto

Lol I don't know if I have a good enough answer for that I guess its coz it rocks, just as hiei said! I'ts really enjoyable too, and you can learn quite a bit of Japanese from it.
Also, what InsanityRanch said, I absolutely agree. There is nobody for me to speak or hear Japanese from, therefore I get my kick out of it from Manga.